The present invention generally relates to a diode unit, and more particularly, to a laser diode for use in electrical or electronic equipment.
Generally, laser diodes are available in a so-called can-seal type, and a unit type. As shown in FIG. 5, the known can-seal type laser diode includes a stem 2 erected on a heat sink 1, a laser diode chip 4 bonded to the stem 2 through a sub-mount 3, a monitor element 8 disposed on the upper surface of the heat sink 1, and a can-seal 5 of a cylindrical shape with a bottom, applied thereover to seal the upper surface of the heat sink 1. Meanwhile, as illustrated in FIG. 6 , the conventional unit type laser diode has a laser diode chip 4 bonded onto a substrate 6 through a sub-mount 3 and having two cleavage faces 4a and 4b, a monitor element 8 formed generally at a central portion of the sub-mount 3, a wiring 14 for supplying power to the laser diode chip 4, another wiring 15 for deriving electric current, formed on the sub-mount 3, through functioning of the monitor element 8, and a flexible circuit 9 connected adjacent to one edge of the substrate 6, with leads 10,11, and 12 being connected to the substrate 6 and wirings 14 and 15 as shown.
In any of the above known two types of laser diodes, it is so arranged to monitor the laser light emitted from one cleavage face by the monitor element 8, thereby to control light output of the laser diode chip 4 so that intensity of the laser light thus monitored reaches a desired level.
More specifically, in the can-seal type laser diode in FIG. 5 as described above, a photo-diode as the monitor element 8 is disposed on the heat-sink 1 so as to receive the laser light directed from the cleavage face at the lower portion of the diode chip 4, while in the unit type laser diode in FIG. 6, the photo-diode 8 for the monitor element is integrally formed on the sub-mount 3 of a silicon material.
In the construction for monitoring the laser output in the conventional laser diodes as described so far, since it is so arranged to receive the laser light emitted from one of the cleavage faces of the laser diode chip by the monitor element through a space therebetween, problems can occur hereinafter described.
Firstly, with respect to the can-seal type laser diode referred to above, modification thereof into an open type without the can-seal 5 may be considered for the requirement of size reduction in an appliance in which the laser diode is to be incorporated. In such a case, it is assumed that the surface of the monitor element 8 is covered by dew formation, dust or the like, and consequently, the laser output as monitor information becomes low in level to apply a high level drive to the laser diode chip 4, thus resulting in breakage of said laser diode chip.
Furthermore, also with respect to the unit type laser diode as described above, if it is formed into an open type, the problem for the faulty monitoring arising from dew formation, dust, etc. and/or problems related to the breakage of the laser diode 4 are also expected. Moreover, particularly in this unit type laser diode, there is such a fundamental problem that, since an incidence angle of the laser light onto the surface of the monitor element 8 (i.e. an angle between an optical axis of the laser light and the surface of the monitor element) is small, the monitor element 8 can not receive a sufficient amount of laser light.